N.M.R. solid echoes in systems of coupled pairs of spins-1/2: II. Thermotropic liquid crystals

Proton solid echo transverse relaxation functions, mapped by measurement of the echo amplitude S y x (t′ = τ) as a function of τ using a P y (90°)-τ-P x (90°)-t′ sequence, are reported for a variety of thermotropic mesogens and mesophases (see Table 1). The relationship of the shapes of this function to the structure and dynamics of the mesogen molecule and the mesophase structure is explored. A heuristic approach is adopted. The spin response for an alkyl chain moiety of a mesogen molecule in a uniaxial mesophase is considered to be the sum of the responses for an assembly of representative subsystems each of which consists of two equivalent pairs of spins-½. The response for the latter is calculated by introducing the effects of motional averaging into the expression obtained in Paper I and estimating an ordering matrix from deuterium quadrupole splittings measured on the corresponding deuteriated mesogen. This model, albeit physically unrealistic, qualitatively reproduces the salient features of the ob...